The present invention relates to a process for producing difluoromethane (hereinafter, also referred to as xe2x80x9cHFC-32xe2x80x9d) having a high purity by removing water by means of distillation from a mixture of HFC-32 comprising water, for example a reaction product comprising HFC-32 and water which product is prepared by a production process for HFC-32 by means of fluorination of dichloromethane (hereinafter, also referred to as xe2x80x9cHCC-30xe2x80x9d).
Recently, the ozone layer depletion of the stratosphere by means of chlorofluorocarbons has been a serious problem, and the uses thereof are prohibited internationally. Further, productions and uses of hydrochlorofluorocarbons are also restricted. HFC-32 as a compound free from chlorine has an ozone sphere destruction factor of zero and thus its global warming factor is small, and has a good freezing capacity, so that HFC-32 is said to be favorable as an alternative cooling medium in place of the chlorofluorocarbons which are restricted.
As a production process of HFC-32, there are, for example, a process in which HCC-30 is fluorinated, and a process in which dichloromethane or/and chlorodifluoromethane are reduced. In these production processes, water (or moisture) is contained in HFC-32 as an aimed product because of water contained in a feedstock and water entrained with a catalyst, as well as water or an alkaline aqueous solution used for removal of unreacted hydrogen fluoride and hydrogen chloride as a by-product.
Generally, a solid dehydration agent such as a zeolite is often used for the removal of water contained a halogenated hydrocarbon. For example, Japanese Patent Kokai Publication No. 7-33695 discloses the use of a dehydrated zeolite for the removal of water in 1,1-dichloro-1-fluoroethane (hereinafter, also referred to as xe2x80x9cHFC-141bxe2x80x9d) and further discloses that HFC-141b is not decomposed by the dehydration agent. When such a solid dehydration agent is used, it is required to have a step and an apparatus to regenerate the dehydration agent. In addition, there occurs a loss of an aimed product which has been adsorbed onto the solid dehydration agent upon the regeneration of the agent.
As to HFC-32, it is required to have a particularly less water content and thus a high purity since it is mostly used as a cooling medium, so that a large amount of a solid dehydration agent is necessary. On the other hand, since a molecular size of HFC-32 is close to that of water, a dehydration ability of the usual solid dehydration agent such as a zeolite is not sufficient, and in addition, HFC-32 is decomposed. Therefore, there is a problem in that that it is difficult to dehydrate HFC-32 compared with the other usual halogenated hydrocarbon.
Thus, the water removal from HFC-32 requires a specific solid dehydration agent, and water removal processes using such solid dehydration agents are proposed in Japanese Patent Kokai Publication Nos. 6-327,968, 7-305,054 and 8-173,799. Those solid dehydration agents are useful when they are used in a closed system as used in a compressor as a cooling medium for an air conditioner (that is, when no water gets into the system from the outside thereof)(See Japanese Patent Kokai Publication No. 7-305,054). However, when water is continuously supplied into a system through a feedstock and so on as in a production step of HFC-32, a dehydration agent is required which has a high performance (a high water adsorption capacity, a high water selectivity on adsorption). When the dehydration agent is regenerated, HFC-32 which has been adsorbed onto the dehydration agent is finally wasted, and therefore, a large amount of HFC-32 is lost if the performance is bad.
In the case in which water is continuously supplied into a system from the outside thereof, water may be removed by reaction of water with lithium chloride. However, this way is only able to lower a water content in HFC-32 to substantially about 500 ppm. Even in this way, lithium chloride has to be regenerated.
It is an object of the present invention to provide a process for producing HFC-32 of which water content is small wherein the water removal from HFC-32 which has been said to be difficult as described above is carried out effectively by means of an easy operation, and in other words to provide a process for refining HFC-32.
When an impurity is separated from the other using a distillation operation, as to an ideal mixture, it is generally said that a boiling point difference between components which constitute the mixture is a measure for an ease extent of the separation. However, a mixture which contains water is largely non-ideal, and separation behaviors of such a mixture are complicated. Thus, as to the water separation, a relative volatility or a vapor-liquid ratio, namely a water content in a vapor phase/a water content in a liquid phase (based on molar concentration) of a system in question becomes important.
For example, a boiling point of trichlorofluoromethane (hereinafter, also referred to as xe2x80x9cCFC-11xe2x80x9d) is about 24xc2x0 C. at atmospheric pressure, which is far lower than 100xc2x0 C. which is a boiling point of water, so that it is expected that a relative volatility of water is considerably smaller than one in a CFC-11/water system. However, when the relative volatility is actually measured, the measurement shows, inversely to the expectation, that the relative volatility is larger than one, that is the water content in the vapor phase is larger than that of the liquid phase.
Such an inversion phenomenon is also observed with respect to dichlorodifluoromethane of which boiling point is further lower (about xe2x88x9230xc2x0 C., and hereinafter, also referred to as xe2x80x9cCFC-12xe2x80x9d). On the other hand, with respect to chlorofluoromethane of which boiling point (about xe2x88x929xc2x0 C., and hereinafter, also referred to as xe2x80x9cHCFC-31xe2x80x9d) is between those of CFC-11 and CFC-12, the relative volatility of water is contrary smaller than one. In addition, even when it is expected in accordance with a difference between boiling points whether the relative volatility of water is larger or smaller than one, there is still a problem in that a figure of the relative volatility itself cannot be expected at all.
For example, in the case of HCFC-31 with which the relative volatility of water is smaller than one, if the system of water-HCFC-31 were ideal, the relative volatility of water is expected to be 0.0073 at a temperature of 25xc2x0 C. by means of calculation in accordance with Raoult""s law. This figure means that it is extremely easy to separate water from HCFC-31 by distillation. However, when the relative volatility of water is measured in such a system, it measured to be about 0.79, which is about one hundred times as large as that of the above expectation, and it is found for the first time that the separation between water and HCFC-31 is no so easy.
What is described above means that how much the relative volatility is larger or smaller than one is unknown based on the difference between boiling points before it is actually measured. When a figure of the relative volatility is unknown, no number of theoretical plates required for the distillation cannot be determined for the separation in the industrial scale. Therefore, it is unknown how many numbers of the theoretical plates in a distillation column is to be used in order to separate water from HFC-32. That is, there occurs a problem in that whether or not water is industrially separated from HFC-32 is unknown.
As described above, when water is contained as a component which constitutes a system, it is not expected at all differently from the usual distillation operation whether a relative volatility of water is larger or smaller than one is determined only based on a boiling point difference, and further how much a figure of the relative volatility is, so that it is also not easy at all to expect water behaviors (for example, whether being concentrated in an enriching section or a stripping section by means of the distillation, the number of necessary plates for the distillation and so on).
Then, the present inventors have measured relative volatilities of water in a water/HFC-32 system, a water/HCFC-31 system and a water/HCC-31 system, intensively studied the results of the measurement as to whether water content in HFC-32 can be separated by the distillation, and found that such separation is possible, so that the present invention has been completed.
For example, the relative volatility of water was measured to be 0.20 at a temperature of 25xc2x0 C. and a pressure of 17 kg/cm2-abs. (absolute pressure) in the water/HFC-32 system. By thus actually measuring the relative volatility of water, it has been expected for the first time that water contained in HFC-32 can be removed while concentrating water in the stripping section by means of the distillation, which has been confirmed.
Therefore, the present invention provides a process for producing a mixture comprising water and HFC-32 in which mixture a water content has been substantially reduced, and preferably for producing a mixture comprising HFC-32 which contains substantially no water characterized in that a mixture comprising water and HFC-32 is subjected to a distillation operation.
In one embodiment of the present invention, the mixture which is subjected to the distillation operation consists substantially of water and HFC-32, and the mixture after the distillation operation consists substantially of HFC-32 of which water content has been reduced and preferably HFC-32 which contains substantially no water.